A received signal strength indicator (RSSI) of enforced electro-magnetic or pure magnetic fields can be used in vehicle access or passive-entry-go (PEG) systems to determine the distance between a transmitter located in the vehicle and a receiving system located in a key fob. The RSSI can be determined by measuring the voltage of a continuous waveform (CW) signal that is transformed from an electro-magnetic or pure magnetic field by an antenna or a coil. If the CW signal is corrupted by distortion the constant envelope of the CW signal starts to vary, leading to inaccurate RSSI information and an inaccurate distance measurement.